Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic:
Net income	$ 29,874	$ 24,523	$ 84,663	$ 71,990
Average common shares outstanding (in shares)	34,174	33,590	34,124	33,566
Basic net income per common share (in dollars per share)	$ 0.87	$ 0.73	$ 2.48	$ 2.14
Diluted:
Adjustment for common share equivalents (in shares)	164	597	192	596
Average common shares outstanding-diluted (in shares)	34,338	34,187	34,316	34,162
Diluted net income per common share (in dollars per share)	$ 0.87	$ 0.72	$ 2.47	$ 2.11
Anti-dilutive shares (in shares)		8		8